Calcium-enriched drinks, i.e., drinks containing edible calcium compounds, are desirable as daily drinks to increase the calcium content of the Japanese, which tend to be calcium deficient.
A known method for elevating the calcium content of a soybean milk drink comprises adding a water soluble calcium salt, such as calcium malate (JP-A-60-47635; the term "JP-A " as used herein means an "unexamined published Japanese patent application"). However, according to this method, when a soybean milk drink containing a large amount of the water soluble calcium salt is thermally sterilized, it becomes cloudy due to the coagulation of proteins, which restricts the sterilization method and the amount of the calcium salt that can be added.
JP-A-52-90662 proposes a method for preventing the coagulation of proteins, which comprises mixing slaked lime with soybean milk, eliminating the solid matters and then treating the residue successively with an acid and an alkali. However, the disadvantage of this method is that it results in large losses due to the treatments and requires a complicated procedure. Although JP-B-2-8689 proposes a method which comprises adding a complex of slaked lime with a saccharide to soybean milk and regulating the pH value with an organic acid, this method requires a complicated procedure and achieves only a small increase in calcium content, i.e., about 0.1% (the term "JP-B " as used herein means an "examined Japanese patent publication").
Also, a method is known which comprises adding a hardly soluble calcium salt, such as calcium lactate, to microcrystalline cellulose (JP-B-5-32009). However, this method suffers from the problem that proteins coagulate at the final step of heat sterilization and thus, an additional homogenization step is required thereafter.
The troubles encountered in these cases apply not only to soybean milk drinks, but also to general milk drinks that are enriched with high amounts of calcium.